The invention relates to a stable crystal form of 5-[(2,4-dioxothiazolidin-5-yl)methyl]-2-methoxy-N-[[4-(trifluoromethyl)phenyl]methyl]benzamide (KRP-297) represented by a formula (1) 
and a process for preparing it.
KRP-297 has very excellent glucose-lowering activity and is a useful compound as an hypoglycemic agent and an insulin sensitizer (M. Nomura et al, Bioorg. Med. Chem. Lett., 9 (1999) 533-538). At the beginning, it was prepared through a process disclosed in Japanese Kokai Hei 9-48771.
The objective is to find out crystals homogeneous and excellent in stability and to establish a process for preparing them, in order to prepare KRP-297 on an industrial scale.
With the study and development of the preparative process of KRP-297, the inventors have found that novel crystals different from those having been obtained through conventional process (Japanese Kokai Hei 9-48771) can be obtained, leading to the completion of the invention. Namely, it has been confirmed that, by additionally recrystallization of the crystals (old form crystals) of KRP-297 obtained through conventional processes (for example, Japanese Kokai Hei 9-48771 etc.) from a suitable solvent, they are converted to more homogeneous and more stable novel crystals than conventional ones.
The novel crystals of KRP-297 are characterized by exhibiting the diffraction angles (2xcex8) at at least 9.7xc2x0, 15.0xc2x0 and 22.5xc2x0 in the X-ray powder diffraction.
The novel crystals of KRP-297 of the present invention can usually be obtained in good reproducibility by recrystallization of the crude crystals obtained after completion of reaction from a suitable solvent.
As the solvents to be used for recrystallization, lower alcohols such as ethanol, water-containing lower alcohols, common organic solvents, mixed solvents if need be, and the like can be mentioned. Preferable solvent is ethanol or isopropyl alcohol.
The novel crystals of the invention have no hygroscopicity and make it possible to supply stably in terms of preparation, which is very advantageous for the industrial production of KRP-297.